Additional capacitor structure and control method

Abstract

The invention discloses an additional capacitor structure and a control method, the direct current side of an inverter is connected in parallel with the additional capacitor structure, capacitor energy is mutually transmitted between a direct current side capacitor and an additional capacitor of the inverter by controlling on / off of a switching tube, and inertia support is provided for alternating current side frequency while the direct current side voltage of the inverter is stabilized. An improved network construction control strategy applied to the inverter is provided based on the energy of the additional capacitor structure, a coupling loop of the additional capacitor energy and the system frequency is constructed, the change of the capacitor energy is positively fed back to the change of the system frequency, and the inertia supporting capacity of the additional capacitor structure is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible DC power transmission, and particularly to an additional capacitor structure and a control method. Background Art

[0002] In recent years, the scale of offshore wind farms has been continuously expanding, and the site selection is developing towards the deep sea and far sea, posing new challenges to the existing power transmission technologies. The high-voltage direct current (VSC-HVDC) technology based on voltage source converters has the characteristics of fast power decoupling control and the ability to access isolated power grids, and has become the preferred solution for offshore wind farms to transmit electric energy to onshore power grids. The converter adopting the grid-following control has low inertia characteristics, which easily leads to too high a rate of change of the grid frequency and a frequency deviation exceeding the safety threshold, thus affecting the safe and stable operation of the system. With the continuous increase in the proportion of offshore wind power access, the low-inertia VSC-HVDC system will further deteriorate the frequency stability of the power grid. Therefore, a control scheme that can provide inertial support for the power grid has become increasingly important in the wind power grid-connected system. Most of the existing grid-forming control technologies of VSCs utilize the inertial energy of DC capacitors. However, currently, the inertial energy of DC capacitors of VSCs is generally insufficient, and the DC voltage offset caused by their charging and discharging will have a negative impact on the operation of other devices in the system. Summary of the Invention

[0003] Object of the Invention: Aiming at the above problems, the present invention proposes an additional capacitor structure and a control method to solve the problems of insufficient energy and DC voltage offset during inertial support in the wind power grid-connected system, and improve the stability of the power system during operation.

[0004] Technical Solution: To achieve the object of the present invention, the present invention proposes an additional capacitor structure, which includes a first IGBT switch tube T 1 , a second IGBT switch tube T 2 , an inductor L, a first capacitor C1, and a second capacitor C2; wherein, the first capacitor C1 is the DC-side capacitor of a three-phase two-level inverter, and the second capacitor C2 is an additional capacitor.

[0005] The positive electrode of the capacitor C1 is connected to the collector of T 1 , the negative electrode of the first capacitor C1 is connected to the positive electrode of the inductor L, and the negative electrode of the inductor L is connected to the emitter of T 1 , forming a first loop;

[0006] The positive electrode of the inductor L is connected to the emitter of T 2 , the negative electrode is connected to the negative electrode of the second capacitor C2 and the emitter of T 1 , and the positive electrode of the second capacitor C2 is connected to the collector of T 2 , forming a second loop, and both ends of the first capacitor C1 are connected to a three-phase two-level inverter.

[0007] In addition, the present invention also proposes a control method based on the above-mentioned additional capacitance structure, and this method includes the following steps:

[0008] In the first step, determine the action range of the additional capacitance structure, and set the action range of the additional capacitance structure as the energy of capacitor C1 > 1.04 p.u. or < 0.96 p.u;

[0009] In the second step, formulate the action process of the first IGBT switch tube T c1 when the energy E of capacitor C1 1 > 1.04 p.u., and the second IGBT switch tube T 2 ; when E c1 is within the range of [0.96 p.u, 1.04 p.u.], the conduction signals of the first IGBT switch tube T 1 and the second IGBT switch tube T 2 are [0, 0]; when E c1 > 1.04 p.u., the first capacitor C1 discharges, and the second capacitor C2 absorbs the excess energy. Turn on the first IGBT switch tube T 1 , and the conduction signals of the second IGBT switch tube T 2 become [1, 1]. When the first IGBT switch tube T 1 conducts, the first capacitor C1 in loop 1 charges L. After L absorbs the energy of C1, it transfers the energy to C2. Set the judgment condition that the energy E of inductor L L ≥ 0.002E c1 . When this condition is satisfied, the conduction signals of the first IGBT switch tube T 1 and the second IGBT switch tube T 2 become [0, 0]. Close T2 to make C2 absorb the energy in its inductor L;

[0010] In the third step, formulate the action process of the first IGBT switch tube T c1 and the second IGBT switch tube T 1 when the energy E of the first capacitor C1 2 < 0.96 p.u.: when E c1 < 0.96 p.u., the first capacitor C1 charges. Turn on the first IGBT switch tube T 1 , and the conduction signals of the second IGBT switch tube T 2 become [0, 1]. When the second IGBT switch tube T 2 conducts, the second capacitor C2 in loop 2 charges L. After L absorbs the energy of C2, it transfers the energy to C1. Set the judgment condition that the energy E of inductor L L ≥ 0.002E c1 ; when this condition is satisfied, the first IGBT switch tube T 1, the conduction signal of the second IGBT switch tube T 2 becomes [1, 0], and the first IGBT switch tube T is closed 1 , and the first capacitor C1 is made to absorb the energy in its inductor L;

[0011] Fourthly, reflect the energy change of the additional capacitor structure to the AC side frequency of the inverter, and realize the positive feedback control of the inverter AC frequency and the energy of the additional capacitor structure.

[0012] Further, the method of step four is as follows:

[0013] Set the equation of the energy stored in the additional capacitor structure and the active power on the DC side / AC side as:

[0014]

[0015] In the formula, C 1 is the capacitance value of the DC side capacitor of the inverter; U dc is the DC side voltage of the inverter; E c is the total energy E of the first capacitor C1 and the second capacitor C2 c = E c1 + E c2 ; P dc is the active power on the DC side of the inverter; P ac is the active power on the AC side of the inverter;

[0016] The power of the synchronous generator has a coupling relationship with the AC frequency, and the equation is as follows:

[0017]

[0018] In the formula, J represents the inertia of the rotor; ω represents the electrical angular velocity of the SG; P m , P e are the mechanical and electromagnetic powers output by the synchronous generator respectively. Let the additional capacitor structure imitate the characteristics of the coupling between the synchronous generator and the AC frequency, and the right sides of the above two equations are combined to obtain:

[0019]

[0020] Integrate both sides of this equation and simplify to get:

[0021]

[0022] In the formula, ω 0 is the rated frequency; Δω is the frequency change; E c0 is the initial total energy of the additional capacitor structure. Ignoring the square term of Δω, the above formula is simplified to:

[0023] J c ωΔω = Ec -E c0 = ΔE c

[0024]

[0025] Introduce E c -ω droop coefficient k c , k c = 1 / J c ω, establish the linear coupling relationship between ω and E c :

[0026] ω = ω 0 + k c (E c - E c0 )

[0027] The calculated AC frequency ω passes through an integration link to obtain the phase θ of the reference voltage, and a feedforward link D is added:

[0028] θ = ∫ωdt + Dω

[0029] The measured reactive power value Q and the reference value Q ref Obtain the reference voltage amplitude U through droop control nref :

[0030] U nref = U n + k q (Q ref - Q)

[0031] In the formula, k q is the droop coefficient, and U n is the amplitude of the inverter AC side voltage;

[0032] Generate the three-phase reference voltage U nref using the reference voltage amplitude U a_ref 、U b_ref 、U c_ref , and the calculation formula is as follows:

[0033]

[0034] The three-phase reference voltage passes through the PWM modulation module to obtain the switching signal of the inverter, and controls the inverter to realize the coupling control of the additional capacitor energy and the AC frequency.

[0035] Furthermore, the calculation method of the said k c is as follows:

[0036] According to the maximum deviation Δω of the grid frequency max and the maximum change value ΔE of the total energy of the additional capacitor structure cmaxDetermine k c value, k c The setting formula of is as follows:

[0037] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0038] The additional capacitor structure and the improved grid-forming control method of the inverter achieve the suppression of the AC frequency change while maintaining the stability of the DC side voltage of the inverter, solve the problems of insufficient energy and DC voltage offset during the inertia support of the wind power grid-connected system, and improve the stability of the power system operation. Brief description of the drawings

[0039] Figure 1 is the structural schematic diagram of the present invention;

[0040] Figure 2 is the method flow chart of the present invention. Detailed implementation manners

[0041] The technical solution of the present invention will be described in detail below, but the protection scope of the present invention is not limited to the described embodiments.

[0042] As Figure 1 shown, the present invention proposes an additional capacitor structure, including a first IGBT switch tube T 1 , a second IGBT switch tube T 2 , an inductor L, a first capacitor C1, and a second capacitor C2; wherein, the first capacitor C1 is the DC side capacitor of a three-phase two-level inverter, and the second capacitor C2 is an additional capacitor.

[0043] The positive electrode of the capacitor C1 is connected to the collector of T 1 , the negative electrode of the first capacitor C1 is connected to the positive electrode of the inductor L, and the negative electrode of the inductor L is connected to the emitter of T 1 , forming a first loop;

[0044] The positive electrode of the inductor L is connected to the emitter of T 2 , the negative electrode is connected to the negative electrode of the second capacitor C2 and the emitter of T 1 , and the positive electrode of the second capacitor C2 is connected to the collector of T 2 , forming a second loop, and both ends of the first capacitor C1 are connected to a three-phase two-level inverter.

[0045] As Figure 2 shown, in addition, the present invention also proposes a control method based on the above additional capacitor structure, and the method includes the following steps:

[0046] Step 1: Determine the operating range of the additional capacitor structure, and set the operating range of the additional capacitor structure as the energy of capacitor C1 > 1.04 p.u. or < 0.96 p.u.;

[0047] Step 2: Develop the operating process of the first IGBT switch tube T c1 when the energy E of capacitor C1 1 > 1.04 p.u., and the second IGBT switch tube T 2 ; when E c1 is within the range of [0.96 p.u., 1.04 p.u.], the conduction signals of the first IGBT switch tube T 1 and the second IGBT switch tube T 2 are [0, 0]; when E c1 > 1.04 p.u., the first capacitor C1 discharges, and the second capacitor C2 absorbs the excess energy. Turn on the first IGBT switch tube T 1 and the second IGBT switch tube T 2 , and the conduction signals change to [1, 1]. When the first IGBT switch tube T 1 conducts, the first capacitor C1 in loop 1 charges L. After L absorbs the energy of C1, it transfers the energy to C2. Set the judgment condition that the energy E of inductor L L ≥ 0.002E c1 . When this condition is met, the conduction signals of the first IGBT switch tube T 1 and the second IGBT switch tube T 2 change to [0, 0], close T2, and let C2 absorb the energy in its inductor L;

[0048] Step 3: Develop the operating process of the first IGBT switch tube T c1 and the second IGBT switch tube T 1 when the energy E of the first capacitor C1 2 < 0.96 p.u.: when E c1 < 0.96 p.u., the first capacitor C1 charges. Turn on the first IGBT switch tube T 1 and the second IGBT switch tube T 2 , and the conduction signals change to [0, 1]. When the second IGBT switch tube T 2 conducts, the second capacitor C2 in loop 2 charges L. After L absorbs the energy of C2, it transfers the energy to C1. Set the judgment condition that the energy E of inductor L L ≥ 0.002E c1 ; when this condition is met, the conduction signals of the first IGBT switch tube T 1 and the second IGBT switch tube T 2 change to [1, 0], and close the first IGBT switch tube T 1, let the first capacitor C1 absorb the energy in its inductor L;

[0049] In the fourth step, reflect the energy change of the additional capacitor structure to the AC side frequency of the inverter, and realize the positive feedback control of the inverter AC frequency and the energy of the additional capacitor structure.

[0050] Furthermore, the method of step four is as follows:

[0051] Set the equation of the energy stored in the additional capacitor structure and the active power on the DC side / AC side as:

[0052]

[0053] In the formula, C 1 is the capacitance value of the DC side capacitor of the inverter; U dc is the DC side voltage of the inverter; E c is the total energy E of the first capacitor C1 and the second capacitor C2 c = E c1 + E c2 ; P dc is the active power on the DC side of the inverter; P ac is the active power on the AC side of the inverter;

[0054] The power of the synchronous generator has a coupling relationship with the AC frequency, and the equation is as follows:

[0055]

[0056] In the formula, J represents the inertia of the rotor; ω represents the electrical angular velocity of the SG; P m , P e are the mechanical and electromagnetic powers output by the synchronous generator respectively. Let the additional capacitor structure imitate the characteristics of the coupling between the synchronous generator and the AC frequency, and combine the right sides of the above two equations to get:

[0057]

[0058] Integrate both sides of this equation and simplify to get:

[0059]

[0060] In the formula, ω 0 is the rated frequency; Δω is the frequency change; E c0 is the initial total energy of the additional capacitor structure. Ignoring the square term of Δω, the above formula is simplified to:

[0061] J c ωΔω = E c - E c0 = ΔE c

[0062]

[0063] Introduce E c -ω droop coefficient k c , k c = 1 / J c ω, establish the linear coupling relationship between ω and E c :

[0064] ω = ω 0 + k c (E c - E c0 )

[0065] The calculated AC frequency ω passes through an integral link to obtain the phase θ of the reference voltage, and a feedforward link D is added:

[0066] θ = ∫ωdt + Dω

[0067] The measured reactive power value Q and the reference value Q ref Obtain the reference voltage amplitude U through droop control nref :

[0068] U nref = U n + k q (Q ref - Q)

[0069] In the formula, k q is the droop coefficient, and U n is the amplitude of the inverter AC side voltage;

[0070] Adopt the reference voltage amplitude U nref and the phase θ to generate the three-phase reference voltage U a_ref , U b_ref , U c_ref , and the calculation formula is as follows:

[0071]

[0072] The three-phase reference voltage passes through the PWM modulation module to obtain the switching signal of the inverter, and controls the inverter to realize the coupling control of the additional capacitor energy and the AC frequency.

[0073] Furthermore, the calculation method of the said k c is as follows:

[0074] Determine the value of k max according to the maximum deviation Δω of the grid frequency cmax and the maximum change value ΔE of the total energy of the additional capacitor structure c , and the setting formula of k c is:

[0075] As described above, although the present invention has been shown and described with reference to particular preferred embodiments, it should not be construed as a limitation on the present invention itself. Various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. An additional capacitor structure, characterized in that: The structure includes a first IGBT switch tube T1, a second IGBT switch tube T2, an inductor L, a first capacitor C1, and a second capacitor C2; wherein, the first capacitor C1 is connected to the three-phase two-level inverter at both ends, and the first capacitor C1 serves as a DC side capacitor of the three-phase two-level inverter; The positive electrode of the capacitor C1 is connected to the collector of the first IGBT switch tube T1, the negative electrode of the first capacitor C1 is connected to the positive electrode of the inductor L, and the negative electrode of the inductor L is connected to the emitter of the first IGBT switch tube T1, forming a first loop; The positive electrode of the inductor L is connected to the emitter of the second IGBT switch tube T2, the negative electrode is connected to the negative electrode of the second capacitor C2 and the emitter of the first IGBT switch tube T1, the positive electrode of the second capacitor C2 is connected to the collector of the second IGBT switch tube T2, forming a second loop, and the second capacitor C2 serves as an additional capacitor.

2. A control method for an additional capacitor structure according to claim 1, characterized in that: The method comprises the following steps: The first step is to determine the action interval of the additional capacitor structure, and set the action interval of the additional capacitor structure to be the capacitance energy of the first capacitor C1>1.04pu or <0.96pu; The second step is to determine the energy E of the first C1 capacitor. c1 >1.04pu, the action flow of the first IGBT switch tube T1 and the second IGBT switch tube T2 is: when E c1 When the switch is in the interval [0.96pu, 1.04pu], the turn-on signals of the first IGBT switch tube T1 and the second IGBT switch tube T2 are [0, 0]; when E c1 When the voltage is higher than 1.04 pu, the first capacitor C1 discharges and the second capacitor C2 absorbs the excess energy, turning on the first IGBT switch tube T1 and the conduction signal of the second IGBT switch tube T2 becomes [1, 1]. When the first IGBT switch tube T1 is turned on, the first capacitor C1 in loop 1 charges L, and L absorbs the energy of C1 and transfers the energy to C2. The energy E of the inductor L is set as the judgment condition. L ≥0.002E c1 When this condition is met, the turn-on signals of the first IGBT switch tube T1 and the second IGBT switch tube T2 become [0, 0], T2 is closed, and C2 absorbs the energy in the inductor L; The third step is to determine the energy E of the first capacitor C1. c1 When E is less than 0.96pu, the action flow of the first IGBT switch tube T1 and the second IGBT switch tube T2 is as follows: c1 When <0.96pu, the first capacitor C1 is charged, and the conduction signals of the first IGBT switch tube T1 and the second IGBT switch tube T2 become [0, 1]. When the second IGBT switch tube T2 is turned on, the second capacitor C2 in loop 2 charges L, and L absorbs the energy of C2 and transfers the energy to C1. The energy E of the inductor L is set as the judgment condition. L ≥0.002E c1 ; When this condition is met, the conduction signals of the first IGBT switch tube T1 and the second IGBT switch tube T2 become [1, 0], closing the first IGBT switch tube T1, allowing the first capacitor C1 to absorb the energy in the inductor L; The fourth step is to reflect the energy change of the additional capacitor structure to the inverter AC side frequency, so as to realize positive feedback control of the inverter AC frequency and the additional capacitor structure energy.

3. The control method according to claim 2, characterized in that: The method of step 4 is as follows: The equations for setting the additional capacitor structure to store energy and the active power on the DC and AC sides are: Where, C1 is the DC side capacitance of the inverter; U dc is the DC side voltage of the inverter; E c is the total capacitance energy E of the first capacitor C1 and the second capacitor C2 c =E c1 +E c2 ;P dc is the active power on the DC side of the inverter; P ac Measure active power for inverter AC; The power of the synchronous generator has a coupling relationship with the AC frequency, and the equation is as follows: Where, J represents the inertia of the rotor; ω represents the electrical angular velocity of SG; P m , P e are the mechanical and electromagnetic powers output by the synchronous generator, respectively. The additional capacitor structure is used to simulate the characteristics of the synchronous generator coupled with the AC frequency. Combining the above two equations, we get: Integrate both sides of the equation and simplify it to get: In the formula, J c is the equivalent inertia of the first capacitor C1 and the second capacitor C2, ω0 is the rated frequency; Δω is the frequency change; E c0 is the initial total energy of the additional capacitor structure, ignoring the square term of Δω, the above formula is simplified to: J c ωδω=E c -E c0 =ΔE c Introducing E c -ω droop coefficient k c , k c =1 / J c ω, then ω and E c The linear coupling relationship between them is: ω=ω0+k c (E c -E c0 ) The calculated AC frequency ω is used to obtain the phase θ of the reference voltage through an integral link, and a feedforward link D is added to improve the control response speed: θ=∫ωdt+Dω Reactive power measurement value Q and reference value Q ref The reference voltage amplitude U is obtained by droop control nref : U nref =U n +k q (Q ref -Q) In the formula, k q is the droop coefficient, U n is the voltage amplitude on the AC side of the inverter; Using reference voltage amplitude U nref Generate three-phase reference voltage U with phase θ a_ref , U b_ref , U c_ref , the calculation formula is as follows: The three-phase reference voltage is modulated by a PWM module to obtain a switching signal of the inverter, and the inverter is controlled to achieve coupling control of the additional capacitor energy and the AC frequency.

4. The control method according to claim 3, characterized in that: The k c The calculation method is as follows: According to the maximum deviation of the grid frequency Δω max The maximum change in the total energy of the additional capacitor structure ΔE cmax Determine k c Value, k c The setting formula is:

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